Method for manufacturing quantum dot

ABSTRACT

A quantum dot manufacturing method comprises (a) dispersing, in a solvent, nano-seed particles whose crystal planes are exposed, and (b) growing semiconductor layers on the exposed crystal planes of the nano-seed particles in the solvent.

FIELD OF THE INVENTION

The present invention relates to a quantum dot manufacturing method anda quantum dot.

BACKGROUND

A substance made as small (fine or thin) as about several nanometers upto less than 20 nanometers in size begins to exhibit physical propertiesdifferent from those in its bulk state. This phenomenon or effect iscalled a (three to one dimensional) carrier confinement effect or aquantum size effect, and a substance that displays such an effect iscalled a quantum dot (quantum wire or quantum well) or a semiconductornanoparticle, for example. The bandgap (optical absorption wavelength oremission wavelength) of a quantum dot can be adjusted by changing thesize (overall size) of the quantum dot.

One usage of a quantum dot containing a semiconductor material is afluorescent material. A quantum dot containing a semiconductor materialcan emit florescence of a particular wavelength when irradiated withhigh-energy light or particle radiation. An area light source can beobtained by evenly distributing quantum dots and causing the quantumdots to emit light.

Quantum dots having a core-shell structure in which a core (nucleus)portion is covered with a shell layer are available (for example,Japanese Laid-open Patent Publication Nos. 2011-076827 and 2012-087220,and Japanese Patent Nos. 4936338 and 4318710). These quantum dots can bemanufactured by, for example, a liquid phase growth method (for example,Japanese Patent Nos. 4936338, 4318710 and 4502758).

SUMMARY

A main object of the present invention is to obtain a quantum dot havinga novel structure. Another object of the present invention is to obtaina quantum dot that has high reliability and excellent efficiency. Yetanother object of the present invention is to efficiently manufacturequantum dots.

According to an aspect of this invention, there is provided a quantumdot manufacturing method comprising (a) dispersing, in a solvent,nano-seed particles whose crystal planes are exposed, and (b) growingsemiconductor layers on the exposed crystal planes of the nano-seedparticles in the solvent.

A quantum dot having a novel structure (flat multilayer structure) and aquantum dot having high reliability and excellent efficiency can beobtained through the present invention. Moreover, quantum dots can bemanufactured more efficiently.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationships between lattice constants ofa ZnOS mixed crystal system, a GaInN mixed crystal system, and an AlInNmixed crystal system, and the energy gap.

FIG. 2 is a graph showing changes in lattice constants of aZnO_(x)S_(1-x) mixed crystal system, a Ga_(1-x)In_(x)N mixed crystalsystem, and an Al_(1-x)In_(x)N mixed crystal system relative to thecontent x.

FIGS. 3A to 3D are schematic cross-sectional views of a quantum dotmanufacturing process according to a first embodiment, and FIGS. 3E and3F are respectively a cross-sectional view and a perspective viewschematically illustrating a modification example.

FIGS. 4A to 4E are schematic cross-sectional views of a quantum dotmanufacturing process according to a second embodiment, and FIG. 4F is aschematic cross-sectional view of a modification example.

FIGS. 5A to 5E are schematic cross-sectional views of a quantum dotmanufacturing process according to a third embodiment.

FIGS. 6A to 6E are schematic cross-sectional views of a quantum dotmanufacturing process according to a fourth embodiment.

FIGS. 7A to 7D are schematic cross-sectional views of a quantum dotmanufacturing process according to a fifth embodiment, FIG. 7E is aschematic cross-sectional view of a quantum dot according to a referenceexample, and FIG. 7F is a schematic cross-sectional view of a quantumdot according to a sixth embodiment.

FIG. 8 is a schematic side view of an example of a reactor.

FIG. 9 is a schematic side view of an example of an etching device.

FIGS. 10A and 10B are schematic side views of examples of pressurereactors.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

Quantum dots in the visible light spectrum used presently are CdSe/ZnS,InP/ZnS, and the like in which a shell having a large energy gap isstacked on a core having a small energy gap. When a core-shell structureis configured by using different compound materials, lattice mismatch(CdSe/ZnS: 11.1%, InP/ZnS: 7.8%) occurs. Lattice mismatch can causedeformation of crystal lattices and degradation of emission efficiencyand reliability.

FIG. 1 is a graph showing the relationships between the latticeconstants of a ZnOS mixed crystal system, a GaInN mixed crystal system,and an AlInN mixed crystal system, and the energy gap. The term “mixedcrystal system” refers to a system that contains a mixed crystalcontaining two end members and intermediate members. The horizontal axisindicates the lattice constant in terms of nanometers (nm), and thevertical axis indicates the energy gap in terms of electron volt (eV).The energy gaps that determine the emission wavelength are ZnO: 3.2 eV,ZnS: 3.8 eV, AlN: 6.2 eV, GaN: 3.4 eV, and InN: 0.64 eV.

In growing hexagonal crystals in the c-axis direction, the a-axislattice constant is used as the lattice constant within the plane ofgrowth. The a-axis lattice constants of ZnO and ZnS are, respectively,0.324 nm and 0.382 nm. The a-axis lattice constants of AlN, GaN, and InNare, respectively, 0.311 nm, 0.320 nm, and 0.355 nm.

In pairing the compounds, a combination with the closest latticeconstants is ZnO having a lattice constant of 0.324 nm and GaN having alattice constant of 0.320 nm, and lattice mismatch exceeding 1% ispresent even with this combination.

FIG. 2 is a graph schematically showing changes in lattice constants (inthe a-axis direction) of the ZnO_(x)S_(1-x) mixed crystal system, theGa_(1-x)In_(x)N mixed crystal system, and the Al_(1-x)In_(x)N mixedcrystal system relative to the content x. The horizontal axis indicatesthe content x, and the vertical axis indicates the lattice constant.ZnOS and AlGaInN have the same hexagonal wurtzite crystal structure.When a mixed crystal is formed, the lattice constant can be adjusted tobe some point between the two end members, and lattice matching can beachieved.

There is a particular relationship established between the latticeconstant, the energy gap, and the content. FIG. 2 shows substantiallythe same information as that of FIG. 1. Which graph is to be used isdetermined according to the parameter focused. For example, the contentsat which lattice matching is achieved are the contents at which thevalues along the vertical axis (lattice constant) are the same in FIG.2. A preferable matching range of lattice matching would be a range inwhich the difference in lattice constant is within ±1.0% with referenceto a smaller lattice constant (100%).

The region in which lattice matching is possible among ZnO_(x)S_(1-x),Al_(1-x)In_(x)N, and Ga_(1-x)In_(x)N is marked by a box. In the contentrange of Al_(1-x)In_(x)N (x: 0.3 to 1.0), Ga_(1-y)In_(y)N (y: 0.15 to1.0), and ZnO_(z)S_(1-z) (z: 0.47 to 1.0), lattice matching among ZnOS,AlInN, and GaInN is possible.

When ZnOS is used as a base crystal and AlGaInN that lattice-matcheswith ZnOS is grown on ZnOS, the strain at the interface can be reduced.Reducing strain prevents crystal defects, and a quantum dot with highefficiency would be achieved. Most crystals have properties that changedepending on the crystal orientation. Obtaining a surface of a specifiedcrystal plane orientation facilitates control of the properties. Anepitaxial layer with controlled properties can be easily grown on a baseparticle when the base particle is processed to have a specified planeorientation.

A quantum dot having high reliability can be manufactured by using ZnOS,which is easy to manufacture, to form a base particle, performingetching to expose a specified plane (a specified crystal plane), andperforming heteroepitaxy of lattice-matching AlGaInN crystals on theexposed surface. It is also possible to further grow other AlGaInNcrystals thereon.

FIG. 3D is a schematic cross-sectional view of a quantum dot 61according to the first embodiment. For example, an In_(0.60)Ga_(0.40)Nlayer 13 having a (0001) surface is formed on a ZnO_(0.72)S_(0.28)nano-seed particle 11 having a specified surface 12, which is a C (0001)plane, and an In_(0.67)Al_(0.33)N layer 15 having a (0001) surface isformed on the In_(0.60)Ga_(0.40)N layer 13. As illustrated in FIG. 2,ZnO_(0.72)S_(0.28), In_(0.60)Ga_(0.40)N, and In_(0.67)Al_(0.33)N arelattice-matched. The bandgap of each crystal satisfies the relationship,In_(0.60)Ga_(0.40)N<In_(0.67)Al_(0.33)N<ZnO_(0.72)S_(0.28), as indicatedin FIG. 1. The In_(0.60)Ga_(0.40)N layer 13 functions as an emissionlayer, and the ZnO_(0.72)S_(0.28) layer 11 and the In_(0.67)Al_(0.33)Nlayer 15 on both sides function as barrier layers that transmit lightemitted by band edge emission caused by carrier recombination. The(0001) plane orientations of the crystals are coincident, and desiredproperties are readily obtained.

Referring now to FIGS. 3A to 3D, an example of a quantum dotmanufacturing method according to the first embodiment is described.Although one quantum dot is described, a large number of quantum dotsare manufactured simultaneously by the liquid phase synthesis describedbelow. The size of the quantum dots can be controlled by adjustingreaction conditions and the like. For example, the average particlediameter can be controlled to be 50 nm or less so that the particles canbe suspended in a liquid phase.

First, a ZnO_(0.72)S_(0.28) base particle is synthesized by hotinjection.

As illustrated in FIG. 8, a 300 ml quartz flask 40 is prepared as areactor. The flask 40 is equipped with an outlet port, a port 16 throughwhich inert gas purging is possible, multiple dedicated ports equippedwith syringes 17 through which reaction precursors can be injected, anda temperature measuring unit 18 equipped with a thermocouple. Argon (Ar)is used as the inert gas. The flask 40 is placed on a heating mantle 19.

The syringes 17 respectively filled with reaction precursors, i.e.,diethylzinc (Zn(C₂H₅)₂) sealed with inert gas, oxygen-bubbled octylamine(C₈H₁₇NH₂), and bis(trimethylsilyl) sulfide, are prepared. To obtain 4.0mmol of diethylzinc, 2.8 mmol of oxygen-bubbled octylamine, and 1.2 mmolof bis(trimethylsilyl) sulfide, 410 μl of diethylzinc, 460 μl ofoctylamine, and 250 μl of bis(trimethylsilyl) sulfide are prepared.Oxygen-bubbled octylamine is prepared in advance by bubbling oxygen intooctylamine for 2 minutes. The composition of the nano-particle can bechanged by changing the ratios of the reaction precursors.

Into the reactor 40, 8 g of tri-n-octylphosphine oxide (TOPO) and 4 g ofhexadecylamine (HDA) serving as a reaction solvent are placed. An inertgas (Ar) atmosphere is created, and the content is heated to 300° C. byusing the heating mantle 19 while stirring with a stirrer to completelydissolve all components.

As soon as the reaction solvent reaches 300° C., the reaction precursorsare quickly injected from the respective syringes. Due to pyrolysis ofthe reaction precursors, crystal nuclei of ZnO_(0.72)S_(0.28) (wurtzite)are generated. Immediately after injection of the reaction precursors,rapid cooling is conducted to drop the temperature to 200° C. If thetemperature is left at 300° C., most of the reaction precursors areconsumed in nucleus formation, and nuclei of various sizes occur withelapse of the time. Rapid cooling can prevent additional nucleusformation in the reaction solvent.

The reaction solvent is then re-heated to 240° C. and retained at theconstant temperature for 240 minutes to grow ZnO_(0.72)S_(0.28). As aresult, nano-sized base particles having a particle diameterdistribution with a 20 nm center size can be synthesized as illustratedin FIG. 3A.

The particle diameter (distribution) of the base particles can beadjusted by controlling the heating time for the reaction solvent. Forexample, when the reaction solvent is re-heated to 240° C. and retainedat the constant temperature for 180 minutes to grow ZnO_(0.72)S_(0.28),nano-sized base particles having a particle diameter distribution with a10 nm center size can be synthesized. The particle diameter ofZnO_(0.72)S_(0.28) is preferably 20 nm or less.

Subsequently, the reactor is left to cool naturally to 100° C., and aheat treatment is performed at 100° C. for 1 hour. This stabilizessurfaces of the base particles. After being cooled to room temperature,the reaction solution is combined with butanol, which is ananticoagulant, to prevent aggregation of the base particles, followed bystirring for 10 hours. Purification is conducted by repeatingcentrifugal separation (4000 rpm, 10 minutes) by alternately usingdehydrated methanol that dissolves the solvent (TOPO) and toluene thatdisperses the base particles. By repeating the centrifugal separation,undesirable raw materials and solvents are completely removed.

As illustrated in FIG. 3B, the ZnO_(0.72)S_(0.28) base particle isetched to form a nano-seed having a (0001) surface (C-plane) as thespecified surface. The ZnO_(0.72)S_(0.28) base particles synthesizedthrough the above-described step are in a dispersed state in methanol.Methanol is evaporated to condense the base particles. However, the baseparticles will aggregate if methanol is completely evaporated; thus, asmall amount of methanol is left to remain, and ultrapure water servingas a photoetching solution is added thereto. In order to selectivelyexpose the (0001) surface, which is the specified plane orientation,nitric acid (61 vol %) is added. The blend ratio of the etching solution21 is ultrapure water:nitric acid (61 vol %)=600:1. Oxygen is bubbledfor 5 minutes into this solution maintained at 25° C.

FIG. 9 is a schematic side view illustrating a structure of aphotoetching device. Light from a light source 27, such as a mercurylamp, is monochromatized with a monochromator 28 and can be incident onthe solution inside the flask 40 through a rod lens 29.

The mixed solution (etching solution) 21 after completion of bubbling isplaced in the flask 40. Light having an emission wavelength of 405 nm(3.06 eV) and a half-value width of 6 nm, which is light having asufficiently shorter wavelength than the absorption edge wavelength ofthe ZnO_(0.72)S_(0.28) base particle, is applied to the etchingsolution. Light from a mercury lamp dispersed with the monochromator isused as the light source. The ZnO_(0.72)S_(0.28) base particles havingvarious absorption edge wavelengths due to differences in particle sizeabsorb light and undergo photodissolution reaction, through which thesurfaces of the particles dissolve in the photodissolution liquid andthe diameter gradually decreases. As etching proceeds, the absorptionedge wavelength shifts toward the short wavelength side. In addition,nitric acid, which has a selective etching effect, assists thephotoetching, and a (0001) crystal plane appears as a result. The lightis applied until the absorption edge wavelength becomes shorter than thewavelength of the applied light and the photodissolution reaction stops.The irradiation time is 20 hours. After completion of etching, thoroughwashing with water is conducted to completely replace the etchingsolution. As a result, ZnO_(0.72)S_(0.28) nano-seed particles dominantlyhaving the (0001) surface (C-plane) can be obtained.

The specified surface is not limited to the C-plane. In order to preparea ZnO_(z)S_(1-z), nano-seed having an M-plane instead of the C-plane,aqua regia is used as the etching solution. In this case, since theetching rate in the m-axis direction is about 100 times faster than thatin the c-axis direction, the surface can be controlled to an M-plane.When off-direction surface control with respect to the C-plane isdesirable, a mixed solution of ethylenediaminetetraacetic acid.2Na andethylenediamine can be used. The nano-seed surface may be modified withsurface-modifying groups such as anions, amines, thiols, and organicpolymers.

As illustrated in FIG. 3C, an In_(0.60)Ga_(0.40)N layer 13 is grown onthe surface of a ZnO_(0.72)S_(0.28) nano-seed 11 having a specifiedsurface, i.e., the C-plane, exposed. Since In_(0.60)Ga_(0.40)Nlattice-matches with ZnO_(0.72)S_(0.28), an epitaxial layer having goodcrystallinity can be grown. The In_(0.60)Ga_(0.40)N layer 13 having aC-plane is grown.

FIG. 10A is a schematic cross-sectional view of an epitaxial growthsystem, with which pressure is controllable, used in this process.

A reactor 32 into which raw materials are to be placed is formed ofstainless steel on the outer side and hastelloy on the inner side. Thereactor 32 has at least two air supply ports 33 and 34, and an exhaustport 35.

The air supply ports 33 and 34 are respectively coupled to, for example,an Ar gas supply source and a N₂ gas supply source through valves, andAr gas and N₂ gas can be supplied from the air supply ports 33 and 34 tothe interior of the reactor 32. A vacuum pump is coupled to the exhaustport 35 through a valve so that the atmosphere (gas) inside the reactor32 can be evacuated. The partial pressure of each gas in the reactor 32,in particular, the partial pressure of the N₂ gas, can be preciselycontrolled by adjusting the valve.

The reactor 32 is equipped with a temperature sensor 38, a heater 37, astirring mechanism 39, etc. The temperature sensor 38 can measure thetemperature of the content inside the reactor 32. The heater 37 can heatthe content. The stirring mechanism 39 (rotary blade) can stir thecontent.

Six milliliters of an aqueous solution containing the ZnO_(0.72)S_(0.28)nano-seeds 11 that dominantly have the (0001) surface formed in the stepillustrated in FIG. 3B is extracted and freeze-dried. Gallium iodide(108 mg, 0.24 mmol), which is a gallium source, indium iodide (165 mg,0.36 mmol), which is an indium source, sodium amide (500 mg, 12.8 mmol),which is a nitrogen source, and hexadecanethiol (380 μl, 1.0 mmol) andzinc stearate (379 mg, 0.6 mol), which are capping agents, are placed inthe reactor 32 containing diphenyl ether (20 ml) serving as a solvent.

The freeze-dried nanoparticles are preliminarily added to the solvent,diphenyl ether, and ultrasonically dispersed. The resulting mixedsolution is rapidly heated to 225° C. and retained at 225° C. for 80minutes. The nitrogen partial pressure during synthesis is adjusted tobe 1500 Torr. Then the reactor is again left to cool naturally to 100°C., and a heat treatment is conducted at 100° C. for 1 hour. As aresult, the surfaces of the nanoparticles are stabilized.

Subsequently, butanol serving as an anticoagulant is added to thereaction solution cooled to room temperature to prevent aggregation ofthe nanoparticles followed by stirring for 10 hours. Lastly,purification is conducted by repeating centrifugal separation (4000 rpm,10 minutes) by alternately using dehydrated methanol that dissolves thesolvent (TOPO) and toluene that disperses the nanoparticles. Undesirableraw materials and solvents are completely removed. All operationsrelated to preparation of the sample according to this process areperformed inside a glovebox by using vacuum-dried (140° C.) tools anddevices.

As illustrated in FIG. 3C, through this procedure, a quantum dot inwhich the In_(0.60)Ga_(0.40)N nitride semiconductor layer 13 having athickness of 4.0 nm is epitaxially grown on the C (0001) surface 12 ofthe ZnO_(0.72)S_(0.28) nano-seed 11 can be obtained. The modifyinggroups on the surface of the synthesized quantum dot may be substitutedwith other surface-modifying groups, such as anions, amines, and organicpolymers, through ligand exchange. Zinc from zinc stearate serving as acapping agent may mix into InGaN, but this case is also referred to asInGaN.

The thickness of the In_(0.60)Ga_(0.40)N nitride semiconductor layer canbe adjusted by controlling the heating time for the mixed solution. Thethickness of the In_(0.60)Ga_(0.40)N nitride semiconductor layer may beadjusted while monitoring the wavelength of the light (fluorescence)emitted from the nitride semiconductor layer.

FIG. 10B is a schematic cross-sectional view of another epitaxial growthsystem that can be used in this process. The growth system may befurther equipped with a light source 41, optical fibers 42 and 43, and aphotodetector 44 in addition to the structure illustrated in FIG. 10A.

The excited light of a particular wavelength emitted from the lightsource 41 is applied to the mixed solution (the emission layersdispersed in the mixed solution, i.e., the In_(0.60)Ga_(0.40)N nitridesemiconductor layers) through the optical fiber 42. The emission layersdispersed in the mixed solution absorb the excited light and emit lighthaving wavelengths corresponding to their thickness. The light emittedfrom the emission layer is guided, through the optical fiber 43, intothe photodetector 44 having a dispersing function.

As the mixed solution is heated, the thickness of the emission layerincreases. When the thickness of the emission layer increases within therange in which the quantum confinement effect is provided, thewavelength of the light emitted therefrom shifts toward the longwavelength side. As soon as the light of the desired wavelength isdetected by the photodetector 44, heating of the mixed solution isstopped. The thickness of the emission layer can be adjusted by thismethod also.

As illustrated in FIG. 3D, an In_(0.67)Al_(0.33)N nitride semiconductorlayer 15 is formed on the In_(0.60)Ga_(0.40)N nitride semiconductorlayer 13. All operations related to preparation of the sample areperformed inside a glovebox by using vacuum-dried (140° C.) tools anddevices.

Six milliliters of an aqueous solution containing particles that containIn_(0.60)Ga_(0.40)N layers 13 dominantly having the C (0001) surface isextracted and freeze-dried. Aluminum iodide (80 mg, 0.20 mmol), which isan aluminum source, indium iodide (185 mg, 0.40 mmol), which is anindium source, sodium amide (500 mg, 12.8 mmol), which is a nitrogensource, and hexadecanethiol (380 μl, 1.0 mmol) and zinc stearate (379mg, 0.6 mol), which are capping agents, are placed in a flask containingdiphenyl ether (20 ml) serving as a solvent.

The freeze-dried nanoparticles are preliminarily added to the solvent,diphenyl ether, and ultrasonically dispersed. The resulting mixedsolution is rapidly heated to 225° C. and retained at 225° C. for 100minutes. The nitrogen partial pressure during synthesis is adjusted tobe 1500 Torr.

Then the reactor is again left to cool naturally to 100° C., and a heattreatment is conducted at 100° C. for 1 hour. As a result, the surfacesof the nanoparticles are stabilized. Subsequently, butanol serving as ananticoagulant is added to the reaction solution cooled to roomtemperature to prevent aggregation of the nanoparticles, followed bystirring for 10 hours. Lastly, purification is conducted by repeatingcentrifugal separation (4000 rpm, 10 minutes) by alternately usingdehydrated methanol that dissolves the solvent (diphenyl ether) andtoluene that disperses the nanoparticles. By repeating the centrifugalseparation, undesirable raw materials and solvents are completelyremoved.

Through this procedure, a nitride quantum dot in which theIn_(0.67)Al_(0.33)N nitride semiconductor layer 15 having a thickness of5 nm is epitaxially grown on the In_(0.60)Ga_(0.40)N layer 13 can beobtained. The thickness of the In_(0.67)Al_(0.33)N nitride semiconductorlayer may be adjusted by controlling the heating time for the mixedsolution. Zinc from zinc stearate may mix into InAlN, but this case isalso referred to as InAlN.

A nitride quantum dot 51 having an average particle diameter of 12 nmcan be formed through the above-described process. The modifying groupson the surface of the synthesized quantum dot may be substituted byother surface-modifying groups, such as anions, amines, and organicpolymers, by ligand exchange.

As illustrated in FIG. 3E, the ZnO_(0.72)S_(0.28) nano-seed 11 may beremoved. This process is optional. By using an etching solution havingdifferent etching rates for the ZnO_(0.72)S_(0.28) nano-seed, theIn_(0.60)Ga_(0.40)N layer, and the In_(0.67)Al_(0.33)N layer,ZnO_(0.72)S_(0.28) is selectively etched. Diluted hydrochloric acid thatcan selectively etch ZnO_(0.72)S_(0.28) is, for example, used as theetching solution. The blend ratio of the etching solution is, forexample, hydrochloric acid (36 vol %):pure water=1:100. After completionof etching, thorough washing with water is conducted to completelyreplace the etching solution.

In order to synthesize nanoparticles having a desired ZnO_(z)S_(1-z),mixed crystal composition, the ratios of the materials constituting thereaction precursors may be appropriately changed. The lattice mismatchratio between the nano-seed formed of ZnO_(0.72)S_(0.28) and theIn_(0.60)Ga_(0.40)N nitride semiconductor layer is substantially zero.In actual cases, fluctuations due to variation in reaction areallowable. The range of the lattice mismatch may be any range within ±1%when a smaller lattice constant is assumed to be 100%. This can berewritten into a composition range, 0.67≤z≤0.78.

The nano-seed with the specified surface exposed by selective etchinghas, for example, a plate shape. However, it is sufficient if at leastone specified surface is exposed. FIG. 3F illustrates aZnO_(0.72)S_(0.28) nano-seed 11 having one specified surface 12 exposed.

Although the ZnO_(z)S_(1-z) mixed crystal composition has been selectedas an example of the nano-seed base particle, the material may be anygroup II-VI semiconductor material, AB (A represents at least oneselected from Zn and Mg and B represents at least one element selectedfrom O, S, Se, and Te). For example, ZnOSSe or ZnOSe can be produced bythe same procedure by substituting all or some part ofbis(trimethylsilyl) sulfide, which is a sulfur-containing material andserves as a material for the seed particle, with a selenium-containingmaterial, tri-n-octylphosphine selenide. Although ZnOS having a wurtzitecrystal structure has been selected as an example of the nano-seed baseparticle, ZnOS having a zinc blende structure may be selected instead.

Furthermore, although an example of using a hot injection method as amethod for preparing the nano-seed base particle is described, asolvothermal method that uses reaction in a high-temperature,high-pressure alcohol solvent may be employed, or the nano-seed baseparticle may be prepared by crushing a bulk member into fine particles(nanosize) by ultrasonic waves and the like. The modifying groups on thesurface of the synthesized nano-seed base particle may be substitutedwith other surface modifying groups, such as anions, amines, thiols, andorganic polymers, by ligand exchange.

Second Embodiment

FIG. 4E is a schematic cross-sectional view of a quantum dot 62according to a second embodiment. For example, an In_(0.67)Al_(0.33)Nlayer 31 having a (0001) surface, an In_(0.60)Ga_(0.40)N layer 13 havinga (0001) surface, and an In_(0.67)Al_(0.33)N layer 15 having a (0001)surface are sequentially stacked on top of each other on aZnO_(0.72)S_(0.28) nano-seed particle 11 having a specified surface 12,which is a C (0001) surface.

Compared to the quantum dot 61 according to the first embodiment, anIn_(0.67)Al_(0.33)N layer 31 is formed between the ZnO_(0.72)S_(0.28)nano-seed particle 11 and the In_(0.60)Ga_(0.40)N layer 13. TheIn_(0.67)Al_(0.33)N layers 31 and 15 are disposed on both sides of theIn_(0.60)Ga_(0.40)N layer 13 so as to constitute a symmetricalstructure.

This is a structure in which three layers, namely, theIn_(0.67)Al_(0.33)N layer 31, the In_(0.60)Ga_(0.40)N layer 13, and theIn_(0.67)Al_(0.33)N layer 15 that have a (0001) surface andlattice-match with the ZnO_(0.72)S_(0.28) nano-seed particle 11 arestacked on the C (0001) surface of the ZnO_(0.72)S_(0.28) nano-seedparticle 11. When the In_(0.60)Ga_(0.40)N layer 13 functions as anemission layer, the In_(0.67)Al_(0.33)N layers 31 and 15 on both sidesfunction as barrier layers.

A quantum dot manufacturing method according to the second embodimentwill now be described with reference to FIGS. 4A to 4E.

FIG. 4A illustrates a process of synthesizing the ZnO_(0.72)S_(0.28)base particle 11, and FIG. 4B illustrates a process of exposing thespecified surface 12 of the base particle 11. These processes arebasically the same as the processes of the first embodiment illustratedin FIGS. 3A and 3B, and the description therefor is omitted.

As illustrated in FIG. 4C, an In_(0.67)Al_(0.33)N layer 31 is formed onthe nano-seed particle 11 having the C (0001) surface 12 exposed. Thisis the same process as the process of growing the In_(0.67)Al_(0.33)Nlayer 15 illustrated in FIG. 3D. All operations related to preparationof the sample are performed inside a glovebox by using vacuum-dried(140° C.) tools and devices.

Six milliliters of an aqueous solution containing the ZnO_(0.72)S_(0.28)nano-seeds 11 dominantly having the C (0001) surface is extracted andfreeze-dried. Aluminum iodide (80 mg, 0.20 mmol), which is an aluminumsource, indium iodide (185 mg, 0.40 mmol), which is an indium source,sodium amide (500 mg, 12.8 mmol), which is a nitrogen source, andhexadecanethiol (380 μl, 1.0 mmol) and zinc stearate (379 mg, 0.6 mol),which are capping agents, are placed in a flask containing diphenylether (20 ml) serving as a solvent. The freeze-dried nanoparticles arepreliminarily added to the solvent, diphenyl ether, and ultrasonicallydispersed.

The resulting mixed solution is rapidly heated to 225° C. and retainedat 225° C. for 40 minutes. The nitrogen partial pressure duringsynthesis is adjusted to be 1500 Torr. Then the reactor is again left tocool naturally to 100° C., and a heat treatment is conducted at 100° C.for 1 hour. As a result, the surfaces of the nanoparticles arestabilized.

Subsequently, butanol serving as an anticoagulant is added to thereaction solution cooled to room temperature to prevent aggregation ofthe nanoparticles, followed by stirring for 10 hours. Lastly,purification is conducted by repeating centrifugal separation (4000 rpm,10 minutes) by alternately using dehydrated methanol that dissolves thesolvent (diphenyl ether) and toluene that disperses the nanoparticles.By repeating the centrifugal separation, undesirable raw materials andsolvents are completely removed. Thus, an In_(0.67)Al_(0.33)N layer 31having a thickness of 2.0 nm can be formed on the nano-seed 11.

As illustrated in FIG. 4D, an In_(0.60)Ga_(0.40)N layer 13 is grown onthe In_(0.67)Al_(0.33)N layer 31. This process is basically the same asthe process illustrated in FIG. 3C in that an In_(0.60)Ga_(0.40)N layeris grown. All operations related to preparation of the sample areperformed inside a glovebox by using vacuum-dried (140° C.) tools anddevices.

Six milliliters of a methanol dispersion containing quantum dots formedof the ZnO_(0.72)S_(0.28) nano-seeds 11 dominantly having a C (0001)surface and the In_(0.67)Al_(0.33)N layers 31 is extracted and injectedto a reactor containing a diphenyl ether (20 ml) serving as a solventtogether with gallium iodide (108 mg, 0.24 mmol), which is a galliumsource, indium iodide (165 mg, 0.36 mmol), which is an indium source,sodium amide (500 mg, 12.8 mmol), which is a nitrogen source, andhexadecanethiol (380 μl, 1.0 mmol) and zinc stearate (379 mg, 0.6 mol),which are capping agents.

The resulting mixed solution is rapidly heated to 225° C. and retainedat 225° C. for 80 minutes. The nitrogen partial pressure duringsynthesis is adjusted to be 1500 Torr. An In_(0.60)Ga_(0.40)N layer 13grows as a result.

Then the reactor is again left to cool naturally to 100° C., and a heattreatment is conducted at 100° C. for 1 hour. As a result, the surfacesof the nanoparticles are stabilized. Subsequently, butanol serving as ananticoagulant is added to the reaction solution cooled to roomtemperature to prevent aggregation of the nanoparticles, followed bystirring for 10 hours. Lastly, purification is conducted by repeatingcentrifugal separation (4000 rpm, 10 minutes) by alternately usingdehydrated methanol that dissolves the solvent (diphenyl ether) andtoluene that disperses the nanoparticles. By repeating the centrifugalseparation, undesirable raw materials and solvents are completelyremoved. Through this procedure, a quantum dot in which theIn_(0.60)Ga_(0.40)N layer 13 having a thickness of 4.0 nm is formed onthe In_(0.67)Al_(0.33)N layer 31 can be obtained.

The modifying groups on the surface of the synthesized quantum dot maybe substituted with other surface modifying groups, such as anions,amines, and organic polymers, by ligand exchange.

As illustrated in FIG. 4E, an In_(0.67)Al_(0.33)N layer 15 is formed onthe In_(0.60)Ga_(0.40)N layer 13. This process is basically the same asthe process for forming the In_(0.67)Al_(0.33)N layer 15 illustrated inFIG. 3D, and the description therefor is omitted. As a result, a quantumdot 52 in which the In_(0.67)Al_(0.33)N 15 having a thickness of 4.0 nmis formed on the In_(0.60)Ga_(0.40)N layer 13 can be obtained.

FIG. 4F illustrates a process for removing the ZnO_(0.72)S_(0.28)nano-seed. This process is optional. Particles each containing theZnO_(0.72)S_(0.28) nano-seed 11, the In_(0.67)Al_(0.33)N layer 31, theIn_(0.60)Ga_(0.40)N layer 13, and the In_(0.67)Al_(0.33)N layer 15 arein a dispersed state in methanol. Methanol is evaporated to condense thenanoparticles. A small amount of methanol is left to remain.

Then the resulting solution is added to an etching solution havingdifferent etching rates for the ZnO_(0.72)S_(0.28) nano-seed, theIn_(0.60)Ga_(0.40)N layer, and the In_(0.67)Al_(0.33)N layer so as toetch-away the ZnO_(0.72)S_(0.28) nano-seed. Diluted hydrochloric acid(36 vol %) that can selectively etch the ZnO_(0.72)S_(0.28) nano-seed isused as the etching solution. The blend ratio of the etching solution ishydrochloric acid (36 vol %):pure water=1:100. After completion ofetching, thorough washing with water is conducted to completely replacethe etching solution. As a result, a nitride quantum dot having anaverage particle diameter of 12 nm, and formed of a multilayer body thatincludes the In_(0.67)Al_(0.33)N layer 31, the In_(0.60)Ga_(0.40)N layer13, and the In_(0.67)Al_(0.33)N layer 15 can be obtained.

Third Embodiment

FIG. 5E is a schematic cross-sectional view of a quantum dot 63according to a third embodiment. This is a structure in which anIn_(0.67)Al_(0.33)N layer 53 (barrier layer) covers a flat-plate-shapedIn_(0.60)Ga_(0.40)N layer 13 (emission layer). The flat-plate-shapedIn_(0.60)Ga_(0.40)N layer 13 can be deemed to be a core layer, and theIn_(0.67)Al_(0.33)N layer 53 can be deemed to be a shell layer.

A quantum dot manufacturing method according to the third embodimentwill now be described with reference to FIGS. 5A to 5E.

FIG. 5A illustrates a process for forming an In_(0.60)Ga_(0.40)N layer13 after the ZnO_(0.72)S_(0.28) base particle 11 is synthesized and thespecified surface 12 of the base particle 11 is exposed. These processesare basically the same as the processes of the first embodimentillustrated in FIGS. 3A to 3C, and the description therefor is omitted.

As illustrated in FIG. 5B, a ZnO_(0.72)S_(0.28) layer 51 (seed layer) isgrown on the In_(0.60)Ga_(0.40)N layer 13. This process is basically thesame as the process illustrated in FIG. 3A in that ZnO_(0.72)S_(0.28) isgrown.

Now, FIG. 8 is referred. The syringes 17 respectively filled withreaction precursors, i.e., diethylzinc (Zn(C₂H₅)₂) sealed with inertgas, oxygen-bubbled octylamine (C₈H₁₇NH₂), and bis(trimethylsilyl)sulfide, are prepared. To obtain 4.0 mmol of diethylzinc, 2.8 mmol ofoxygen gas-bubbled octylamine, and 1.2 mmol of bis(trimethylsilyl)sulfide, 410 μl of diethylzinc, 460 μl of octylamine, and 250 μl ofbis(trimethylsilyl) sulfide are prepared. Oxygen-bubbled octylamine isprepared in advance by bubbling oxygen into octylamine for 2 minutes.

A methanol dispersion (reaction solvent) containing quantum dots eachformed of the ZnO_(0.72)S_(0.28) nano-seed 11 dominantly having a C(0001) surface and the In_(0.60)Ga_(0.40)N layer 13 is placed in areactor 40. An inert gas (Ar) atmosphere is created, and the dispersionis heated to 300° C. with the heating mantle 19 while being stirred witha stirrer.

As soon as the reaction solvent reaches 300° C., the reaction precursorsare quickly injected from the respective syringes. Immediately afterinjection of the reaction precursors, rapid cooling is conducted to dropthe temperature to 200° C. The reaction solvent is then re-heated to240° C. and retained at the constant temperature for 20 minutes to growZnO_(0.72)S_(0.28). As a result, the ZnO_(0.72)S_(0.28) layer 51 havinga thickness of 2.0 nm as illustrated in FIG. 5B can be formed. TheZnO_(0.72)S_(0.28) layer 51 dominantly has a C (0001) surface.

Subsequently, the reactor is left to cool naturally to 100° C., and aheat treatment is performed at 100° C. for 1 hour. After being cooled toroom temperature, the reaction solution is combined with butanol, whichis an anticoagulant, followed by stirring for 10 hours. Purification isconducted by repeating centrifugal separation (4000 rpm, 10 minutes) byalternately using dehydrated methanol and toluene.

As illustrated in FIG. 5C, an In_(0.60)Ga_(0.40)N layer 13 a layer isagain grown on the ZnO_(0.72)S_(0.28) layer 51. This process isbasically the same as the process for forming the In_(0.60)Ga_(0.40)Nlayer 13 illustrated in FIG. 3C (or FIG. 5A), and the descriptiontherefor is omitted. Note that after forming the In_(0.60)Ga_(0.40)Nlayer 13 a, a ZnO_(0.72)S_(0.28) layer (seed layer) and anIn_(0.60)Ga_(0.40)N layer (emission layer) may further be formed.

As illustrated in FIG. 5D, the ZnO_(0.72)S_(0.28) nano-seed 11 and theseed layer 51 are removed. This process is basically the same as theprocess for removing ZnO_(0.72)S_(0.28) illustrated in FIGS. 3E and 4F.

The particles each containing the ZnO_(0.72)S_(0.28) nano-seed 11, theIn_(0.60)Ga_(0.40)N layer 13, the ZnO_(0.72)S_(0.28) seed layer 51, andthe In_(0.60)Ga_(0.40)N layer 13 a are in a dispersed state in methanol.Methanol is evaporated to condense the nanoparticles. A small amount ofmethanol is left to remain.

Then the resulting solution is added to an etching solution havingdifferent etching rates for ZnO_(0.72)S_(0.28) (nano-seed 11 and seedlayer 51) and In_(0.60)Ga_(0.40)N (emission layers 13 and 13 a) so as toetch away ZnO_(0.72)S_(0.28). Diluted hydrochloric acid that canselectively etch ZnO_(0.72)S_(0.28) is used as the etching solution. Theblend ratio of the etching solution is, for example, hydrochloric acid(36 vol %):pure water=1:100. After completion of etching, thoroughwashing with water is conducted to completely replace the etchingsolution. As a result, the In_(0.60)Ga_(0.40)N layers 13 and 13 aseparate, and single-layer bodies respectively formed ofIn_(0.60)Ga_(0.40)N layers 13 and 13 a can be obtained.

As illustrated in FIG. 5E, each of the single-layer bodies,In_(0.60)Ga_(0.40)N layers 13 and 13 a, is coated with anIn_(0.67)Al_(0.33)N layer 53. This process is basically the same as theprocess for forming the In_(0.67)Al_(0.33)N layers 15 and 31 illustratedin FIGS. 3D and 4C in that In_(0.67)Al_(0.33)N is grown.

Six milliliters of an aqueous solution containing the single-layerbodies, In_(0.60)Ga_(0.40)N layers 13 and 13 a, dominantly having the(0001) surface is extracted and freeze-dried. Aluminum iodide (80 mg,0.20 mmol), which is an aluminum source, indium iodide (185 mg, 0.40mmol), which is an indium source, sodium amide (500 mg, 12.8 mmol),which is a nitrogen source, and hexadecanethiol (380 μl, 1.0 mmol) andzinc stearate (379 mg, 0.6 mol), which are capping agents, are placed ina flask containing diphenyl ether (20 ml) serving as a solvent. Thefreeze-dried nanoparticles are preliminarily added to the solvent,diphenyl ether, and ultrasonically dispersed.

The resulting mixed solution is rapidly heated to 225° C. and retainedat 225° C. for 100 minutes. The nitrogen partial pressure duringsynthesis is adjusted to be 1500 Torr. Then the reactor is again left tocool naturally to 100° C., and a heat treatment is conducted at 100° C.for 1 hour.

Subsequently, butanol serving as an anticoagulant is added to thereaction solution cooled to room temperature, followed by stirring for10 hours. Lastly, purification is conducted by repeating centrifugalseparation (4000 rpm, 10 minutes) by alternately using dehydratedmethanol that dissolves the solvent (diphenyl ether) and toluene thatdisperses the nanoparticles. As a result, an In_(0.67)Al_(0.33)N layer53 having a thickness of 5 nm and covering the In_(0.60)Ga_(0.40)N layer13 or 13 a can be formed.

In the manufacturing methods according to the first embodiment and thesecond embodiment, the number of emission layers provided on onenano-seed particle is one (single layer). However, in the manufacturingmethod according to the third embodiment, the number of emission layersprovided on one nano-seed particle is two (or more than two). Since itis considered that the planar size of the emission layers provided onthe same nano-seed particle is substantially the same, the overallvariation in planar size of the emission layers is considered to be lessin the manufacturing method according to the third embodiment. In themanufacturing methods of the embodiments, the step of exposing aspecified crystal plane of a seed particle (the step illustrated in FIG.3B, 4B, or 5A) takes a particularly long time and more efforts; thus,the manufacturing method according to the third embodiment with which alarger number of emission layers can be obtained in one step ofprocessing (photo-etching) the nano-seed particles can be considered tobe more productive and efficient.

Fourth Embodiment

FIG. 6E is a schematic cross-sectional view of a quantum dot 64according to a fourth embodiment. The quantum dot has a structure inwhich a flat-plate-shaped multilayer body formed of anIn_(0.67)Al_(0.33)N layer 31 and an In_(0.60)Ga_(0.40)N layer 13 iscovered with an In_(0.67)Al_(0.33)N layer 53. The multilayer body thatincludes the In_(0.67)Al_(0.33)N layer 31 and the In_(0.60)Ga_(0.40)Nlayer 13 can be deemed to be a core layer, and the In_(0.67)Al_(0.33)Nlayer 53 can be deemed to be a shell layer.

Compared to the quantum dot 63 according to the third embodiment, thecore layer has a two-layer structure that includes theIn_(0.67)Al_(0.33)N layer 31 and the In_(0.60)Ga_(0.40)N layer 13.Because the In_(0.67)Al_(0.33)N layer 31 is provided, the thickness ofthe overall shell layer (barrier layer) including theIn_(0.67)Al_(0.33)N layer 53 can be made more uniform.

A quantum dot manufacturing method according to the fourth embodimentwill now be described with reference to FIGS. 6A to 6E.

FIG. 6A illustrates a process of forming a multilayer body that includesthe In_(0.67)Al_(0.33)N layer 31 and the In_(0.60)Ga_(0.40)N layer 13after synthesizing the ZnO_(0.72)S_(0.28) base particle 11 and exposingthe specified surface 12 of the base particle 11. This process isbasically the same as the process of the second embodiment illustratedin FIGS. 4A to 4D, and the description therefor is omitted.

As illustrated in FIG. 6B, a ZnO_(0.72)S_(0.28) layer 51 (seed layer) isformed on the In_(0.60)Ga_(0.40)N layer 13. This process is basicallythe same as the process for forming the ZnO_(0.72)S_(0.28) layer 51illustrated in FIG. 5B, and the description therefor is omitted.

As illustrated in FIG. 6C, the In_(0.67)Al_(0.33)N layer 31 a and theIn_(0.60)Ga_(0.40)N layer 13 a are again grown on the ZnO_(0.72)S_(0.28)layer 51. This process is basically the same as the process for formingthe In_(0.67)Al_(0.33)N layer 31 and the In_(0.60)Ga_(0.40)N layer 13illustrated in FIGS. 4C and 4D (and FIG. 6A), and the descriptiontherefor is omitted. Note that, after the In_(0.60)Ga_(0.40)N layer 13 ais formed, a ZnO_(0.72)S_(0.28) layer (seed layer) and a multilayer bodythat includes an In_(0.67)Al_(0.33)N layer and an In_(0.60)Ga_(0.40)Nlayer may be further formed.

As illustrated in FIG. 6D, the ZnO_(0.72)S_(0.28) nano-seed 11 and theseed layer 51 are removed. This process is basically the same as theprocess of removing ZnO_(0.72)S_(0.28) illustrated in FIG. 5D, and thedescription therefor is omitted. A multilayer body that includes anIn_(0.67)Al_(0.33)N layer 31 or 31 a and an In_(0.60)Ga_(0.40)N layer 13or 13 a can be obtained.

As illustrated in FIG. 6E, the multilayer body including theIn_(0.67)Al_(0.33)N layer 31 or 31 a and the In_(0.60)Ga_(0.40)N layer13 or 13 a is covered with an In_(0.67)Al_(0.33)N layer 53. This processis basically the same as the In_(0.67)Al_(0.33)N covering processillustrated in FIG. 5E, and the description therefor is omitted.

Fifth Embodiment

FIG. 7D is a schematic cross-sectional view of a quantum dot 65according to a fifth embodiment. This has the same structure as thequantum dot illustrated in FIG. 4F. That is, the quantum dot has athree-layer multilayer structure that includes an In_(0.67)Al_(0.33)Nlayer 31, an In_(0.60)Ga_(0.40)N layer 13, and an In_(0.67)Al_(0.33)Nlayer 15. When the In_(0.60)Ga_(0.40)N layer 13 functions as an emissionlayer, the In_(0.67)Al_(0.33)N layers 31 and 15 on both sides functionas barrier layers.

A quantum dot manufacturing method according to the fifth embodimentwill now be described with reference to FIGS. 7A to 7D.

FIG. 7A illustrates a process of forming an In_(0.67)Al_(0.33)N layer 31after synthesizing the ZnO_(0.72)S_(0.28) base particle 11 and exposingthe specified surface 12 of the base particle 11. This process isbasically the same as the process of the second embodiment illustratedin FIG. 4A to 4C, and the description therefor is omitted.

The largest width within the specified surface 12 of the nano-seed 11is, for example, about 7 nm. The particle diameter and the longest widthwithin the specified surface 12 of the nano-seed 11 are preferably 20 nmor less.

The thickness of the In_(0.67)Al_(0.33)N layer 31 is about 2.0 nm. Thisthickness is enough for providing a quantum confinement effect.

As illustrated in FIG. 7B, the ZnO_(0.72)S_(0.28) nano-seed 11 isremoved. This process is basically the same as the process of removingZnO_(0.72)S_(0.28) illustrated in FIG. 4F (or FIG. 5D or 6D), and thedescription therefor is omitted. As a result, a single-layer body,In_(0.67)Al_(0.33)N layer 31, is obtained.

As illustrated in FIG. 7C, an In_(0.60)Ga_(0.40)N layer 13 is grown onthe In_(0.67)Al_(0.33)N layer 31. This process is basically the same asthe process of growing the In_(0.60)Ga_(0.40)N layer 13 illustrated inFIG. 4D, and the description therefor is omitted. As a result, a quantumdot in which the In_(0.60)Ga_(0.40)N layer 13 having a thickness of 4.0nm is formed on the In_(0.67)Al_(0.33)N layer 31 can be obtained. Thisthickness is enough for providing a quantum confinement effect.

As illustrated in FIG. 7D, an In_(0.67)Al_(0.33)N layer 15 is formed onthe In_(0.60)Ga_(0.40)N layer 13. This process is basically the same asthe process of forming the In_(0.67)Al_(0.33)N layer 15 illustrated inFIG. 4E (or FIG. 3D), and the description therefor is omitted. As aresult, a quantum dot in which the In_(0.67)Al_(0.33)N layer 15 having athickness of 2.0 nm is formed on the In_(0.60)Ga_(0.40)N layer 13 can beobtained.

The timing of removing the nano-seed 11 may be any time after formationof the In_(0.67)Al_(0.33)N layer 31. For example, the nano-seed 11 maybe removed after formation of the In_(0.60)Ga_(0.40)N layer 13 or afterformation of the In_(0.67)Al_(0.33)N layer 15.

The In_(0.67)Al_(0.33)N layer 31 (first barrier layer) and theIn_(0.67)Al_(0.33)N layer 15 (second barrier layer) have a wider bandgapthan the In_(0.60)Ga_(0.40)N layer 13 (first emission layer), and aquantum confinement effect (one-dimensional quantum confinement effect)occurs in the first emission layer 13 in at least the thicknessdirection. In the fifth embodiment, the thickness of the first emissionlayer 13 is about 4 nm, and the wavelength of the light emittedtherefrom is about 550 nm.

The size of the first emission layer 13 (and the first and secondbarrier layers 31 and 15) in the planar direction is defined by the sizeof the specified surface 12 of the nano-seed 11. The thickness of thefirst emission layer 13 is easily adjustable (by controlling the heatingtime for the mixed solution in the step of growing theIn_(0.60)Ga_(0.40)N layer 13). That is, the size of the first emissionlayer 13 can be one-dimensionally (only in the thickness direction)adjusted. Since the largest width within the specified surface 12 of thenano-seed 11 is preferably 20 nm or less, the largest width of the firstemission layer 13 (and the first and second barrier layers 31 and 15) inthe in-plane direction is also preferably 20 nm or less.

FIG. 7E illustrates a quantum dot 91 according to a reference example.The quantum dot 91 has a core-shell structure, and includes a sphericalcore 93 that has a light-emitting property, and a shell layer 95 thatcovers the core 93 and has a wider bandgap than the core 93. A(three-dimensional) quantum confinement effect supposedly occurs in thecore 93. The size of the spherical core 93 is difficult to controlone-dimensionally and typically controlled three-dimensionally bychanging the particle size (diameter). The emission layer(In_(0.60)Ga_(0.40)N layer 13) of the embodiments can be deemed tocorrespond to the core of the reference example, and the barrier layer(In_(0.67)Al_(0.33)N layer 15 or 31) of the embodiments can be deemed tocorrespond to the shell layer of the reference example.

In the fifth embodiment, the size of the emission layer 13 can beadjusted only one-dimensionally (only in the thickness direction). Thus,precise adjustment of the size of the emission layer 13 is relativelyeasy, and it is possible to more precisely control the bandgap andemission wavelength of the emission layer 13. Meanwhile, in thereference example, the size of the core 93 can only be adjustedthree-dimensionally. Thus, it is relatively difficult to preciselycontrol the size of the core 93, and it is difficult to preciselycontrol the bandgap and emission wavelength of the core 93.

The combination of the In_(0.67)Al_(0.33)N layers 31 and 15 thatfunction as barrier layers and the In_(0.60)Ga_(0.40)N layer 13 thatfunctions as an emission layer constitutes a type-I band off-setstructure. A type-I band off-set structure has a structure in which alayer (In_(0.60)Ga_(0.40)N layer) with a narrow bandgap is sandwiched bylayers (In_(0.67)Al_(0.33)N layers) with wide bandgaps, and carrierrecombination can occur inside the layer with a narrow bandgap.

The In_(0.60)Ga_(0.40)N layer that functions as an emission layer can bereplaced by a ZnO_(0.72)S_(0.28) layer. The combination of theIn_(0.67)Al_(0.33)N layer and the ZnO_(0.72)S_(0.28) layer constitutes atype-II band off-set structure. In the type-II band off-set structure,carrier recombination can occur between adjacent layers.

A modification example of the quantum dot manufacturing method accordingto the fifth embodiment will now be described. In this modificationexample, the In_(0.60)Ga_(0.40)N layer 13 of the quantum dot illustratedin FIG. 7D is replaced by a ZnO_(0.72)S_(0.28) layer 14.

As in the process illustrated in FIG. 7A, after a ZnO_(0.72)S_(0.28)base particle 11 is synthesized and the specified surface 12 of the baseparticle 11 is exposed, an In_(0.67)Al_(0.33)N layer 31 is formed. Then,as in the process illustrated in FIG. 7B, the ZnO_(0.72)S_(0.28)nano-seed 11 is removed to obtain a single-layer body,In_(0.67)Al_(0.33)N layer 31.

Subsequently, as illustrated in FIG. 7C, a ZnO_(0.72)S_(0.28) layer 14is grown on the In_(0.67)Al_(0.33)N layer 31. This process is basicallythe same as the process illustrated in FIGS. 3A and 4A in that aZnO_(0.72)S_(0.28) layer is grown.

FIG. 8 is now referred. A methanol dispersion (reaction solvent)containing single-layer bodies, In_(0.67)Al_(0.33)N layers 31,dominantly having the (0001) surface is placed in the reactor 40. Aninert gas (Ar) atmosphere is created, and the dispersion is heated to300° C. with the heating mantle 19 while being stirred with a stirrer.

As soon as the reaction solvent reaches 300° C., the reaction precursors(diethylzinc, octylamine, and bis(trimethylsilyl) sulfide) are quicklyinjected from the respective syringes 17. Due to pyrolysis of thereaction precursors, crystal nuclei of ZnO_(0.72)S_(0.28) are generated.Immediately after injection of the reaction precursors, rapid cooling isconducted to drop the temperature to 200° C. The reaction solvent isthen re-heated to 240° C. and retained at the constant temperature for70 minutes to grow ZnO_(0.72)S_(0.28). As a result, theZnO_(0.72)S_(0.28) layer 14 having a thickness of 7.0 nm is synthesized.The ZnO_(0.72)S_(0.28) layer 14 dominantly has a C (0001) surface.

The thickness of the ZnO_(0.72)S_(0.28) layer can be adjusted bycontrolling the heating time for the reaction solvent. For example, whenthe reaction solvent is re-heated to 240° C. and retained at theconstant temperature for 50 minutes to grow ZnO_(0.72)S_(0.28), aZnO_(0.72)S_(0.28) layer 14 having a thickness of 5.0 nm can be formed.

Subsequently, the reactor is left to cool naturally to 100° C., and aheat treatment is performed at 100° C. for 1 hour. After being cooled toroom temperature, the reaction solution is combined with butanol, whichis an anticoagulant, to prevent aggregation of ZnO_(0.72)S_(0.28),followed by stirring for 10 hours. Purification is conducted byrepeating centrifugal separation (4000 rpm, 10 minutes) by alternatelyusing dehydrated methanol and toluene.

Subsequently, as illustrated in FIG. 7D, an In_(0.67)Al_(0.33)N layer 15is grown on the ZnO_(0.72)S_(0.28) layer 14. This process is basicallythe same as the process illustrated in FIG. 4C in that anIn_(0.67)Al_(0.33)N layer is grown on the C (0001) surface ofZnO_(0.72)S_(0.28). The process is also basically the same as theprocess illustrated in FIG. 3D (or FIG. 4E) in that anIn_(0.67)Al_(0.33)N layer is grown.

Six milliliters of an aqueous solution containing the quantum dots eachformed of the In_(0.67)Al_(0.33)N layer 31 and the ZnO_(0.72)S_(0.28)layer 14 is extracted and freeze-dried. Aluminum iodide (80 mg, 0.20mmol), which is an aluminum source, indium iodide (185 mg, 0.40 mmol),which is an indium source, sodium amide (500 mg, 12.8 mmol), which is anitrogen source, and hexadecanethiol (380 μl, 1.0 mmol) and zincstearate (379 mg, 0.6 mol), which are capping agents, are placed in aflask containing diphenyl ether (20 ml) serving as a solvent. Thefreeze-dried nanoparticles are preliminarily added to the solvent,diphenyl ether, and ultrasonically dispersed.

The resulting mixed solution is rapidly heated to 225° C. and retainedat 225° C. for 40 minutes. The nitrogen partial pressure duringsynthesis is adjusted to be 1500 Torr. Then the reactor is again left tocool naturally to 100° C., and a heat treatment is conducted at 100° C.for 1 hour.

Subsequently, butanol serving as an anticoagulant is added to thereaction solution cooled to room temperature, followed by stirring for10 hours. Lastly, purification is conducted by repeating centrifugalseparation (4000 rpm, 10 minutes) by alternately using dehydratedmethanol and toluene that disperses the nanoparticles. As a result, anIn_(0.67)Al_(0.33)N layer 15 having a thickness of 2.0 nm can be formedon the ZnO_(0.72)S_(0.28) layer 14.

When the thickness of each of the In_(0.67)Al_(0.33)N layers 31 and 15is about 2 nm and the thickness of the ZnO_(0.72)S_(0.28) layer 14 isabout 7 nm, the emission wavelength is about 2.9 μm. When the thicknessof the ZnO_(0.72)S_(0.28) layer 14 is about 5 nm, the emissionwavelength is about 2.5 μm.

Sixth Embodiment

FIG. 7F is a schematic cross-sectional view of a quantum dot 66according to a sixth embodiment. This is obtained by further stacking anIn_(0.60)Ga_(0.40)N layer 55 (second emission layer) and anIn_(0.67)Al_(0.33)N layer 57 (third barrier layer) on the quantum dot 65according to the fifth embodiment illustrated in FIG. 7D. The processfor growing the In_(0.60)Ga_(0.40)N layer 55 and the In_(0.67)Al_(0.33)Nlayer 57 is basically the same as that of the fifth embodimentillustrated in FIGS. 7C and 7D. Alternatively, an emission layer and abarrier layer may be further stacked to manufacture a quantum dot thatincludes three or more emission layers.

The multilayer body (first flat multilayer structure) that includes thefirst barrier layer 31, the first emission layer 13, and the secondbarrier layer 15 constitutes one emission unit. The multilayer body(second flat multilayer structure) that includes the second barrierlayer 15, the second emission layer 55, and the third barrier layer 57constitutes another emission unit. The second barrier layer 15 and thethird barrier layer 57 have a wider bandgap than the second emissionlayer 55. A quantum confinement effect (one-dimensional quantumconfinement effect) occurs in the second emission layer 55 in at leastthe thickness direction.

When the thickness of the second emission layer 55 is equal to thethickness (4 nm) of the first emission layer 13, the emission intensity(wavelength: 550 nm) can be ideally made twice as large as the emissionintensity of the quantum dot 65 (FIG. 7D). When the thickness (forexample, 5 nm) of the second emission layer 55 is different from thethickness (4 nm) of the first emission layer 13, light of multiplewavelengths (wavelength of 550 nm and wavelength of 630 nm) can beemitted.

As a modification example of the quantum dot 66, the In_(0.60)Ga_(0.40)Nlayers 13 and 55 that function as emission layers may be respectivelyreplaced by ZnO_(0.72)S_(0.28) layers 14 and 56. For example, when thethickness of each of the barrier layers 31, 15, and 57 formed ofIn_(0.67)Al_(0.33)N layers is 2 nm and when the thickness of theZnO_(0.72)S_(0.28) layer 14 and the thickness of the ZnO_(0.72)S_(0.28)layer 56 are, respectively, 5 nm and 7 nm, 2.5 μm light and 2.9 μm light(infrared) can be emitted.

Although the present invention has been described with reference toembodiments, the present invention is not limited by these embodiments.The range of lattice matching indicated in the embodiments is merelyillustrative, and multilayer structures can be freely combined as longas the content is adjusted to achieve lattice matching. Moreover,various components and materials may be appropriately changed dependingon the manufacturing conditions, usage of quantum dots, etc. It isobvious for a person skilled in the art that various other alterations,improvements, combinations, etc., are possible.

What are claimed are:
 1. A quantum dot manufacturing method comprising:(a) dispersing, in a solvent, nano-seed particles whose crystal planesare exposed; and (b) growing flat-plate-shaped first nitridesemiconductor layers epitaxially on the exposed crystal planes of thenano-seed particles in the solvent, the first nitride semiconductorlayers being made of In_(x)(Al_(m)Ga_(n))_(1-x)N (0.15≤x≤1.0, m+n=1.0).2. The quantum dot manufacturing method according to claim 1, whereinthe step (a) comprises: (a1) preparing base particles made of groupII-VI semiconductor material containing Zn or Mg as group II element;and (a2) forming the nano-seed particles by etching the base particlesin liquid phase.
 3. The quantum dot manufacturing method according toclaim 2, wherein, in the substep (a2), the base particles are etched bya selective photoetching process.
 4. The quantum dot manufacturingmethod according to claim 3, wherein the nano-seed particles are formedby etching the base particles with an etching solution containing nitricacid, C-planes of the nano-seed particles being exposed.
 5. The quantumdot manufacturing method according to claim 3, wherein the nano-seedparticles are formed by etching the base particles with an etchingsolution containing aqua regia, M-planes of the nano-seed particlesbeing exposed.
 6. The quantum dot manufacturing method according toclaim 2, wherein, in the substep (a2), each crystal plane of thenano-seed particles has a maximum width in an in-plane direction of 20nm or less.
 7. The quantum dot manufacturing method according to claim2, wherein, in the substep (a1), the base particles contain ZnOS.
 8. Thequantum dot manufacturing method according to claim 1, wherein, in thestep (b), second nitride semiconductor layers made ofIn_(y)(Al_(p)Ga_(q))_(1-y)N (0.15≤y≤1.0, p+q=1.0) and having differentcomposition from the first nitride semiconductor layers are epitaxiallygrown on the first nitride semiconductor layers.
 9. The quantum dotmanufacturing method according to claim 8, wherein, in the step (b),third nitride semiconductor layers having the same composition as thefirst nitride semiconductor layers are epitaxially grown on the secondnitride semiconductor layers.
 10. The quantum dot manufacturing methodaccording to claim 1, wherein, in the step (b), first semiconductorlayers, seed layers having the same composition as the nano-seedparticles, and second semiconductor layers are sequentially epitaxiallygrown on the crystal planes of the nano-seed particles, wherein themethod further comprises (c) removing the nano-seed particles and theseed layers to separate the first and second semiconductor layers fromeach other, and wherein the first semiconductor layers include the firstnitride semiconductor layers made of In_(x)(Al_(m)Ga_(n))_(1-x)N(0.15≤x≤1.0, m+n=1.0).
 11. The quantum dot manufacturing methodaccording to claim 10, further comprising (d) covering each of the firstand second semiconductor layers, which are separated from each other,with third semiconductor layers.
 12. The quantum dot manufacturingmethod according to claim 11, wherein each of the second semiconductorlayers includes a layer having a composition corresponding to the firstnitride semiconductor layers made of In_(x)(Al_(m)Ga_(n))_(1-x)N(0.15≤x≤1.0, m+n=1.0).
 13. The quantum dot manufacturing methodaccording to claim 12, wherein each of the first and secondsemiconductor layers further includes a second nitride semiconductorlayer made of In_(y)(Al_(p)Ga_(q))_(1-y)N (0.15≤y≤1.0, p+q=1.0) andhaving different composition from the first nitride semiconductor layer.14. The quantum dot manufacturing method according to claim 13, whereinthe third semiconductor layer has the same composition as the firstnitride semiconductor layers.
 15. The quantum dot manufacturing methodaccording to claim 1, wherein in the step (b), first semiconductorlayers having plate shapes with thicknesses such that a quantumconfinement effect occurs are epitaxially grown on the crystal planes ofthe nano-seed particles, wherein the method further comprises: (c)removing the nano-seed particles after growing the first semiconductorlayer; and (d) sequentially epitaxially growing second and thirdsemiconductor layers, having plate shapes with thicknesses such that aquantum confinement effect occurs, on main surfaces of the firstsemiconductor layers, and wherein the first semiconductor layers includethe first nitride semiconductor layers made ofIn_(x)(Al_(m)Ga_(n))_(1-x)N (0.15≤x≤1.0, m+n=1.0).